Well, I can confirm it's NOT related to modest. I flashed the newest firmware (51-3), restored my home directory from the procedure above, plus the stuff from my latest backup (i.e., repositories, etc), and then ran apt-get update, apt-get upgrade. At this point, no modest-related packages were installed. I forgot to list all the packages that were updated, but libfarsight and rtcomm-related packages were among those listed. After the updates, the N800 goes into a white-screen reboot loop, again.

From this thread I suspect rtcomm is the problem.